The answer in this question is 3, but I wonder why 1 can't be an answer. Doesn't 1's "one" mean an unspecific one like in 3?

Q. Which is the same usage as "one" in the paragraph? - 3
1. I don't know which one is better
2. He is one of the most famous people
3. I love ice cream. Please buy me one.
4. One should be careful while using this knife.
5. I bought one bag and two pairs of shoes.
ex)Today's modern woman isn't waiting for a fairy tale ending to get the ring of her dreams...But while many still refuse to put a diamond on their left hand, they like the idea of wearing one on their right hand...